Carburetors for two-stroke or four-stroke engines are arranged in an inlet duct, which extends to the engine body itself. The carburetor often has two air valves, one throttle valve and one choke valve. The throttle valve is used for governing the inlet air flow while the choke valve is used for cold starts. The choke valve is then usually entirely closed, either manually or automatically. A small opening in the choke valve sees to that only a small amount of air passes the throttle valve in conjunction with the starting attempt. This results in a substantial underpressure in the inlet duct by the nozzles located downstreams. Thereby, a great amount of fuel is supplied and a very rich fuel mixture is obtained. This is necessary since a substantial amount of fuel condenses on the walls of the inlet duct, the crankcase and the combustion chamber under these conditions. There are also carburetors with only one air valve, which then functions as a throttle valve as well as a choke valve. Furthermore, there are fuel injection systems, which have a separate starting system comprising a choke valve and a starting nozzle, functioning in accordance with carburetor principles. The amount of fuel for starting is then substantially influenced by the underpressure from the choke valve.
The characteristic of these fuel supply systems is that, at cold start, an air valve, usually called a choke valve, is almost entirely closed, so that a substantial underpressure is formed downstreams the air valve. Often the choke valve closes entirely and a small opening or a small hole allows for the necessary bypassing of a small amount of air. The size of the small air opening is dimensioned according to the lowest starting temperature by which the product is usually used. For a chain saw this can for instance be -25 degrees C. This implies that at higher temperatures, e.g. +20 degrees C., the air opening is too small to give a correct air/fuel mixture for starting. This leads to an unnecessarily rich fuel mixture and to a more difficult or absent starting function. There is also a great risk that the engine stops immediately after start. Furthermore, the rich fuel mixture leads to an unnecessary soot formation in the engine and unnecessarily high exhaust emissions. For, when starting the product, the throttle control is pulled out entirely, since it is hardly possible to fine-adjust the control for optimum air opening in real life usage.